A common type of offshore system for producing hydrocarbons, includes a vessel floating at the sea surface and having a turret rotatably mounted about a vertical axis on the vessel hull, and a group of catenary chains extending at downward inclines to the seafloor. The vessel weathervanes (turns to head in different directions) with changing winds, waves, and currents. The turret is prevented from rotating with the vessel by the fact that when the turret turns, horizontal components of force applied by the chains are in directions that do not pass through the axis of rotation, and these horizontal components therefore apply a torque tending to restore the turret to its initial or quiescent rotational orientation.
Friction in the bearings that rotatably support the turret on the vessel hull result in the turret initially turning with the vessel away from the quiescent turret position. Such misalignment (with respect to the quiescent turret position) continues until the torque applied by the mooring chains overcomes the bearing friction. If the misalignment angle at which the turret begins to turn back is large, then such misalignment can cause wear in chain attachment points, twist in flow lines, and large unexpected dynamic turret rotations. Unexpected dynamic turret rotation occurs when large alignment torque and consequent forces applied by the anchor chains, first exceed static friction of the bearings and begin turret rotation. Once the turret begins to rotate, the bearings apply only dynamic friction which is much less than static friction, resulting in rapid turret rotation. Significant turret misalignment (from its quiescent orientation) is especially likely in large and heavy turrets that have large diameter bearing systems. While it is possible to provide an electric motor that turns the turret when sensors detect misalignment, it is generally preferred that a nonactive system be used for greater reliability. A mechanism that could increase the torque applied by mooring chains or other chain devices (e.g. cables as well as chains), to help turn a turret back towards its quiescent orientation, would be of considerable value.